In recent years, hybrid vehicles that employ an efficient combination of an engine and an electric motor for traveling have been practically used in view of environmental issues. The hybrid vehicle is equipped with a rechargeable power storage unit, which supplies an electric power to the motor generator for producing a drive power during start and acceleration, and also collects kinetic energy of the vehicle as an electric power during downhill travel and braking.
For the above hybrid vehicles, there has been proposed a structure in which the power storage unit mounted on the vehicle can be charged with an electric power supplied from an external power supply such as a commercial power. When the power storage unit is charged in advance by the external power supply as described above, the vehicle can travel, e.g., for a commute or shopping with its engine in non-operation if a distance traveled is relatively short. Therefore, total fuel consumption efficiency can be enhanced. This travel mode is also referred to as an “EV (Electric Vehicle) travel mode”.
For enhancing the travel performance in the EV travel mode, it is desired to enhance a charge/discharge performance of the power storage unit. A structure employing a plurality of power storage units is already proposed as one of ways for enhancing the charge/discharge performance of the power storage units. In such structure, an electric power converting unit such as a converter for controlling the charge/discharge current of each power storage unit is arranged corresponding to each power storage unit. The purpose of this arrangement is to maintain an appropriate SOC (State Of Charge) value in each power storage unit by charging and discharging each power storage unit independently of the others, and thereby to avoid overcharge and overdischarge.
As an example of a structure that can perform the independent charge/discharge on each power storage unit, Japanese Patent Laying-Open No. 2002-010502 has disclosed a charge/discharge apparatus for storage batteries (power storage units) that can simultaneously charge and discharge a plurality of storage batteries.
As a method for estimating the SOC of each power storage unit, there is a known method using the fact that a specific relationship is present between the SOC of the power storage unit and an open-end voltage of the power storage unit. More specifically, according to this method, an open-end voltage of a target power storage unit is measured, and the SOC corresponding to the measured open-end voltage is determined with reference to relationship characteristics that have been obtained experimentally.
However, in a nickel hydrogen battery or the like that is a typical example of the power storage unit mounted on the hybrid vehicle, changes in open-end voltage are relatively small when the SOC is in a practical range. More specifically, the changes in open-end voltage of the power storage unit are small as compared with the changes in SOC of that power storage unit. Therefore, it is impossible to achieve sufficiently high estimation accuracy only by measuring the open-end voltage.
For increasing the estimation accuracy of the SOC, such a methodology is often employed that the SOC obtained by measuring the open-end voltage as described above is corrected based on an integrated value of the charge/discharge quantity of the power storage unit and this process is successively performed.
However, when the successive correction is performed based on the integrated value of the charge/discharge quantity of the power storage unit, this results in a problem that a deviation from the actual SOC gradually occurs due to an error in sensor or the like.